The present invention relates to a method of detecting remaining capacity for secondary batteries wherein nickel hydroxide is used as a positive electrode active material, such as a nickel metal-hydride secondary battery and a nickel cadmium secondary battery.
In appliances in which secondary batteries are used as a power source, it is necessary to detect the remaining capacity of the secondary batteries in order to prevent overdischarge and overcharge, so that the appliances are used within a suitable range of the remaining capacity of the batteries.
The remaining capacity of a secondary battery can be indexed by the state of charge (SOC) which indicates the rate of accumulated electrical quantities in relation to battery capacity. This SOC can be determined from voltage because charge/discharge characteristic curves corresponding to charge/discharge electric current and temperature can be determined through experimentation. That is, by detecting voltage at a certain electric current value and temperature, and by utilizing the charge/discharge characteristic curves corresponding to the above electric current value and temperature, the SOC of the battery can be obtained.
Accordingly, it is possible to detect, by measuring voltage of a secondary battery, whether the SOC after a final discharge (i.e., the last discharge before the battery is recharged) has reached a permissible lower limit (an SOC of 20%, for example). A determination is then made in accordance with the results whether further discharge will induce an overdischarged state, whereupon discharge can be stopped and the battery charged.
The determination voltage (also referred to as end of discharge voltage) for determining whether the SOC has reached a permissible lower limit as described above has usually been determined in the prior art as follows. The determination voltage was determined based on a discharge characteristic curve (complete discharge characteristic curve) indicative of the relationship between the SOC and voltage that is obtained at the time of complete discharge (first discharge). By way of example, in the case of the discharge characteristic curve A shown by solid lines in FIG. 1, the voltage value 1.237V that corresponds to the SOC of 20% was defined to be the determination voltage for determining whether the SOC has reached the permissible lower limit.
However, in the case of nickel metal-hydride or nickel-cadmium secondary batteries using nickel hydroxide as a positive electrode material, as the charge and incomplete discharge cycles are repeated, a memory effect is observed wherein, as shown in the discharge characteristic curves in FIGS. 2 and 3, voltage corresponding to the SOC decreases in accordance with the number of charge/discharge cycles (represented by [1].about.[10]), and drops considerably, particularly at the final discharge. Thus, if the determination voltage is defined based on the complete discharge characteristic curve as above, there is a problem that it may be erroneously determined that the SOC has reached the permissible lower limit (for example, an SOC of 20%) although there actually remains a sufficient amount of remaining capacity (an SOC of 30%, for example) in the case where charge and discharge are repeated a large number of times as shown in FIG. 3.